In electrophotography an image comprising an electrostatic field pattern, usually of non-uniform strength, (also referred to as an electrostatic latent image) is formed on an insulative surface of an electrophotographic element. The insulative surface comprises a photoconductive layer and an electrically conductive substrate. The electrostatic latent image may be formed by imagewise photo-induced dissipation of the strength of portions of an electrostatic field of uniform strength previously formed on the insulative surface. Typically, the electrostatic latent image is then developed into a toner image by contacting the latent image with a toner composition generally containing a pigment. The toner image is then transferred onto a transfer medium such as paper and fixed thereon by heating and/or pressure. The last step involves cleaning residual toner from the electrophotographic element.
Dry toner compositions used in electrophotography are divided into one-component compositions composed of a toner generally comprising a binder resin having a colorant dispersed therein and two-component compositions composed of a toner and a carrier. Charge control agents are generally melt mixed with the toner resin to control the level and range or distribution of charge of the toner during use. The desired level of chargeability is dependent on the method of development and development hardware. Charge control agents are often selected to achieve the desired charge level. High positive charge levels are often desired in electrophotography. It is also desired to have a narrow range of charge on the toner particles to avoid wrong sign toner particles and also to improve performance of the toner composition. Extraparticulate inorganic fine particles such as fumed silica are frequently added to toner compositions to improve fluidity and anti-caking properties.
Monomeric charge control agents are known in the art such as amines and quaternary ammonium salts for positive charge toners and metal complex dyes for negative charge toners. These charge control agents are generally blended into the melted toner resin which is then cooled and ground into fine particles. Unfortunately, monomeric charge control agents exhibit fast charge decay after prolonged use. Polymeric charge control agents do not exhibit this fast decay after prolonged use. It is therefore desired to utilize polymeric charge control agents in toner particles to improve charge stability.
Hitachi Japanese patent 2120865 dated May 8, 1990 discloses color toner beads comprising resin, conductive polyaniline and carbon black as a charge control agent. The carbon black is weakly charging and therefore the toner will have a low average positive charge and further, will have a broad range of charged particles making the toner susceptible to wrong sign toner particles. Therefore, there still is a need in the art for a suitable polymeric charge control agent for use in positively charged toner compositions.
It is therefore an object of the present invention to provide an improved toner composition comprising an improved polymeric charge control agent.
Other objects and advantages will become apparent from the following disclosure.